static LinkedHashSet<S> ai_matrix_categoriesForTopic(S topic) {
  //ret ai_texts(topic, "$Y", "$X");
  new LinkedHashSet<S> out;
  for (Pair<S> p : ai_texts_a(topic)) {
    if (swic(p.a, "under ")) continue;
    S rel = substring(p.a, lastIndexOf(p.a, ' ')+1);
    if (eqicOneOf(rel, "is", "is not"))
      out.add(p.b);
  }
  out.addAll(ai_texts_quoted("$X", "is a matrix category for", topic));
  ret out;
}